1. Field of the Invention
The present invention relates to a multi-channel optical attenuator and manufacturing method thereof, and more particularly, to a multi-channel optical attenuator in which an actuator structure and a support structure are bonded to each other by a bonding medium in a Planar Lightwave Circuit (PLC) structured optical attenuator to control a light path using a waveguide, and a manufacturing method thereof.
2. Description of the Related Art
An optical attenuator, which is one of main parts in light transmission, is provided with an attenuating part. The attenuating part of the optical attenuator generates light loss of a predetermined amount and outputs an attenuated light signal through an output terminal. In light communication network, optical power received at a predetermined portion is different depending on the system's construction due to a difference in transmission loss of optical fiber according to the transmission distance, a difference in the number of connection points of optical fibers, optical coupling used in the transmission path, and the like. It is the optical attenuator to function to control the above factors.
The optical attenuator is configured to include an optical fiber part provided with an input terminal and an output terminal, and an attenuating part functioning to attenuate optical signals. Also, in case of a multi-channel structure, due to increase in device size and difficulty in fine alignment of optical fiber, there is sometimes used a structure where semiconductor device processing technologies are employed to fabricate a waveguide array of silica or the like and the respective waveguides are moved to adjust the optical transmission amount.
Optical attenuators are classified into fixed optical attenuator and variable optical attenuator according to the variation in the attenuated amount. Also, the variable optical attenuators can be classified into single channel VOA and multi-channel VOA according to the number of the input and output terminals.
FIG. 1 illustrates a structure of a PLC type multi-channel optical attenuator. Optical signal outputted from an optical fiber 110 of an input terminal passes through an optical attenuator 100 between the optical fiber 110 of the input terminal and an optical fiber 120 of an output terminal. The optical attenuator 100 is divided into a fixed waveguide part 130 connected to the optical fiber and a movable waveguide part 140 between the fixed waveguide parts 130. The movable waveguide 140 is arranged adjacent to an actuator 150 positioned at a side portion and is moved in a horizontal direction by the operation of the actuator 150, thereby adjusting the amount of optical signals transferred from the fixed waveguide 130. In FIG. 1, there is shown the multi-channel structure where a plurality of channels each being configured to include the optical fiber 110 of the input terminal, the optical fiber 120 of the output terminal, and the optical attenuator 100 arranged between the optical fibers 110 and 120.
To drive the optical attenuator shown in FIG. 1, there is essentially requested the actuator 150 arranged between the fixed waveguides 130, for moving the movable waveguide 140. Since silica forming the waveguide is formed on a silicon substrate, it is required to fabricate an actuator of a silicon structure. Also, to enable a precise etching and enable the operation of the actuator without any problem, it is necessary to form the thickness of the silicon membrane used as the actuator as thin as approximately 100 μm or less and to provide a support structure for supporting the actuator.
Accordingly, in case of the conventional art, as the support structure for supporting the silicon membrane, glass or silicon is generally used. This support structure of glass or silicon is bonded with the silicon membrane thereby to fabricate an optical attenuator including the actuator. Especially, since glass is transparent and has a relatively low junction temperature with silicon, it facilitates alignment of the waveguide and the actuator when being bonded with the silicon membrane. Thus, it is frequently used as the support structure.
FIGS. 2A to 2D illustrate a manufacturing method of an optical attenuator using glass as the support structure according to the conventional art (Steps a to d).
In FIG. 2A, optical waveguides 220 are formed on a silicon substrate 210. The optical waveguides 220 of a desired number of channels are arranged spaced apart by a predetermined interval from one another. (Step a)
In FIG. 2B, a support structure 230 for supporting the silicon substrate 210 on which the waveguides are formed is prepared. As the support structure 230, transparent glass is used as aforementioned. Cavities 240 where the optical waveguides 220 are inserted and positioned are formed in the support structure 230 of glass. (Step b)
In FIG. 2C, the silicon substrate 210 on which the optical waveguides are formed in the step a (as shown in FIG. 2A) is bonded to the support structure 230 such that the optical waveguides 220 are positioned in the cavities 240. The silicon substrate 210 is made into a thin membrane. (Step c)
In FIG. 2D, the bonded silicon substrate 210 is selectively etch-processed to form an actuator. (Step d)
Thus, in the conventional manufacturing method of the optical attenuator, the silicon substrate 210 is bonded to the support structure 230 of glass by an anodic bonding method in which high voltage and heat are applied. In this bonding process, heat of 400–500° C. is applied, which has an influence on the waveguides formed on the silicon substrate 210. Also, the bonding may be poor depending on the surface state of the bonding surfaces of the silicon substrate and the support structure.
In addition, in the conventional optical attenuator, the support structure has to use a special glass having the same heat expansion coefficient as silicon, which causes the increase of production costs and the difficulty in selecting material.
Further, the conventional method of manufacturing the optical attenuator uses isotropic etching process or sand blasting process to form the cavities in the support structure. However, the conventional manufacturing method makes it difficult to form the cavities having a precise size adapted for the size of the waveguides and it also needs a design to provide a sufficient margin between the channels upon considering the bonding area, which acts as great difficulties in integration and miniaturization of products.